I did Kenya Airways early last year to Livingstone (BHX-CDG-NBO-LVI) and TBH it was fine. Nairobi is a lot calmer and better signed than it was, say 10 years ago. In terms of cleanliness, I would say that it is probably on a par with most regional European Airports and better than some of the Med airports are likely to be in August! It has a number of coffee bars/cafes and a couple of shops selling touristy tat. I was in Business but the pricing was very competitive.
I then took the train from Livingstone to Cape Town so can't really comment on it as a departure point for the return journey.
I haven't done Kigali for a couple of years but last time I did, it was an absolute shambles and not one I would recommend for a family looking for a smooth transit.
I'd take NBO over options like JNB any day.